1. Field of the Invention
The present invention relates generally to optical fiber cable and in particular, to an indoor optical fiber cable without a central strength member.
2. Description of the Related Art
As a result of development in the industry and growing demands for more information at higher speed, an era of fiber to the home (FTTH) in has come to play. Various methods have been used to lay an indoor optical fiber cable in a building. Among the methods, a method of directly pulling cable, connecting the indoor optical fiber cable to existing copper cable, and laying the indoor optical fiber cable by pulling the copper cable out have been used.
While the copper serves as a strength member (SM) in the copper cable, optical fibers cannot play a major role of the SM in the optical fiber cable, thus requiring an additional central strength member (CSM) or SM.
FIG. 1 is a sectional diagram of a conventional optical fiber cable 100 including a CSM 110. As shown, the optical fiber cable 100 includes the CSM 110 deployed in the center, a plurality of optical fiber elements 120 wound in a spiral shape around the CSM 110, sheath 140 deployed in the outermost of the optical fiber cable 100 to envelop the CSM 110 and the optical fiber elements 120, and a SM 130 filled in a space inside the sheath 140 to surround the CSM 110 and the optical fiber elements 120.
However, since the optical fiber cable 100 uses steel wire or fiberglass reinforced plastic (FRP) having a very high elastic modulus as the CSM 110, it is difficult to bend the optical fiber cable 100 including the CSM 110, thereby requiring a very high pull tension in the installation environment or disabling the installation of the optical fiber cable 100. As such, it is common for the optical fiber cable not to include a CSM as an installation route of the optical fiber cable is complex and coarse.
FIGS. 2A and 2B illustrate a conventional indoor optical fiber cable 200 without a CSM. In particular, FIG. 2A is a sectional diagram of the indoor optical fiber cable 200, and FIG. 2B is a side view of the indoor optical fiber cable 200.
Referring to FIGS. 2A and 2B, the indoor optical fiber cable 200 includes a plurality of optical fiber elements 210, which are optical transmission media, sheath 230 deployed in the outermost of the indoor optical fiber cable 200 to envelop the optical fiber elements 210, and a peripheral strength member (PSM) 220 filled in a space inside the sheath 230 to surround the optical fiber elements 210.
However, the sheath 230 of the indoor optical fiber cable 200 may be easily stretched when a strong pull tension is applied during the installation or when the indoor optical fiber cable 200 is stuck in an installation route.
FIGS. 3A and 3B illustrate a stretched state of the indoor optical fiber cable 200. In particular, FIG. 3A is a sectional diagram of the stretched indoor optical fiber cable 200, and FIG. 3B is a side view of the stretched indoor optical fiber cable 200.
Referring to FIGS. 3A and 3B, when the sheath 230 is stretched, an inside diameter of the indoor optical fiber cable 200 is reduced, thus reducing a space between the optical fiber elements 210. At this time, since the optical fiber elements 210 is compressed by the SM 220 and the sheath 230, an increase of an optical loss may be caused, and if there exists an additional stress from the outside, the optical loss may be more increased.
In addition, if the sheath 230 contracts due to a drop in temperature in a state of the stretched sheath 230, the inside diameter of the sheath 230 is reduced further, thus increasing the optical loss of the optical fiber elements 210.
As described above, for the typical indoor optical fiber cable 200 without a CSM, an information transmission characteristic can be deteriorated due to the stretch effect of the sheath 230.